Cataracts occur when the crystalline lens of the eye becomes cloudy. The cataracts may be in both eyes, and may cause fading vision and eventual blindness. Cataracts are the most common cause of vision loss in people over age 40, and the condition is the principal cause of blindness in the world.
Cataracts must be surgically removed to eliminate the cloudiness and restore vision. During cataract surgery, the cloudy natural lens is removed and replaced with a clear artificial lens called an intraocular lens, which is inserted into the lens capsule through a small incision.
As developments have been made in the field of small-incision cataract surgery, the size of the surgical incision has been decreased to promote shorter healing times, in particular, to enable out-patient surgeries. To deliver an intraocular lens through such a small incision, the lens material should be foldable and exhibit favorable shape recoverability. Accordingly, in addition to having optical properties necessary for a lens, soft materials that are flexible and foldable have been investigated for suitability in producing intraocular lenses.
Acrylic materials have a high refractive index and will slowly unfold after being implanted into the eye to recover the original lens shape, and thus are desirable candidates for polymeric lens materials. However, as the shape recoverability of the lens material is increased, the elongation percentage is generally reduced. A material with poor elongation percentage is fragile and easily tears, and thus cannot be reliably folded and unfolded for delivery via a small incision site without cracking or tearing. Accordingly, shape recoverability and elongation percentage are competing properties.
As an example of an acrylic lens material, a polymer can be obtained by polymerizing a hydrophilic monomer including a hydroxyl group-containing alkyl (meth) acrylate, a (meth) acrylamide monomer, and N-vinyl lactam to produce a polymer having a water absorption rate of 1.5 to 4.5% by mass (for example, refer to JP-A-11-56998). Although this material is flexible, ester linkages in the polymer structure undergo hydrolysis, and the hydrolyzate can then be eluted from the lens, potentially causing contamination and chemical irritation of the surrounding eye structure.